Magnesium and its alloys have found a variety of industrial applications. However, because of the reactivity of magnesium and its alloys, and their tendency toward corrosition and environmental degradation, it is necessary to provide the surfaces of this metal with an adequate corrosion-resistant and protective coating. Where articles of magnesium or its alloys are used for decorative purposes, the protective coatings applied thereto must be both decorative and corrosion resistant.
The protection of metallic surfaces, including magnesium and its alloys, against corrosion and actions of the elements, has received considerable attention over the years. Some protection has been afforded the metal by coating its surfaces with paint or enamel. Although such coatings are fairly resistant to chemical attack, they are subject to degradation at high temperatures and adhere poorly to the metal surface particularly when experiencing temperature variations.
In order to provide a more effective and permanent protective coating on magnesium and its alloys, the metal has been anodized in a variety of electrolytic solutions. While anodization of magnesium and its alloys imparts a more effective coating than painting or enameling, still the resulting coated metal has not been entirely satisfactory for its intended applications. The coatings often lack the desired degree of hardness, smoothness, durability, adherence and/or imperviousness required to meet the ever-increasing industrial and household demands.
There is a plethora of prior art patents which deal with anodizing magnesium and its alloys. The following is a list of patents which is representative of the efforts of the prior art workers in this field: U.S. Pat. Nos. 1,574,289; 1,574,290; 2,196,161; 2,197,611; 2,203,670; 2,261,960; 2,276,286; 2,305,669; 2,313,753; 2,313,754; 2,313,756; 2,314,341; 2,321,948; 2,322,205; 2,322,208; 2,322,487; 2,338,924; 2,348,826; 2,414,090; 2,426,254; 2,456,931; 2,766,199; 2,778,789; 2,880,148; 3,477,921; 3,620,939; 3,732,152; 3,791,942; 4,184,926; and 4,227,976. While this list is by no means exhaustive, a review of these patents highlights the significant role which the electrolytic solution plays in the anodizing process and in providing the surface of magnesium and its alloys with the desired coating. Thus, in general, the nature and properties of the coating which is formed on aluminum and its alloys depends, to a great extent, on the composition of the anodic bath (electrolytic solution) used in anodizing the metal. Other parameters such as the process conditions used during the electrodeposition process also contribute to the nature and quality of the coating.
In one early patent, i.e., U.S. Pat. No. 1,574,289, a protective coating for magnesium was provided by immersing the metal, which served as the anode, in a solution of hydrofluoric acid and passing a current therethrough at an applied voltage of about 110 volts or higher. The coating formed on the surface of the metal was believed to be magnesium fluoride or oxy-fluoride.
Later, as disclosed in U.S. Pat. No. 2,313,753, it was found that the coatings produced by treatment with hydrofluoric acid alone as aforesaid are unsatisfactory because they are subject to considerable deterioration when exposed to either the atmosphere or aqueous salt solutions. Accordingly, the latter patent recommended that after subjecting the magnesium article to the action of the fluoride, the resulting coated article must be further treated by subjecting it to the action of a bath containing an arsenic compound in order to alter the fluoride-formed coating to increase its corrosion resistance. The dangers of working with arsenic, however, is well known. Besides, this method requires two separate baths and two separate treatments.
A two-step method of providing a protective coating for magnesium and its alloys is also described in U.S. Pat. No. 2,322,208. According to this patent, the magnesium article is first subjected to the action of a fluoride solution and, in a next step, the coated article is immersed in an aqueous solution of a salt of an oxy-acid of an element selected from the group consisting of chromium, molybdemum, phosphorus, selenium, titanium, tugnsten, vanadium, especially the alkali metal and ammonium salts of such oxy-acids.
U.S. Pat. No. 2,322,487 also discloses that when magnesium or its alloys are treated with acid fluoride solution, the resulting coating is subject to deterioration. This patent, too, requires a post-treatment of the fluoride-treated magnesium or its alloys. According to this patent, after treating the metal with an acid fluoride solution, the coated metal is treated, in a separate step, with an aqueous solution of a soluble alkali, or alkali earth metals, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, barium hydroxide, and the like.
Even as recently as U.S. Pat. No. 4,184,926 which issued on Jan. 22, 1980 to Otto Kozak (the inventor of the present application), the protective coating on magnesuim and its alloys was formed by separate treatments of the metal; first in a solution of hydrofluoric acid to form a fluoro-magnesium layer on the metal, and then, in a spearate step, by immersing the coated metal in an aqueous solution of an alkali metal silicate, and applying 150 to 350 volts between said coated metal, serving as the anode, and a second metal which serves as a cathode.
While the coating produced by the said Kozak patent exhibits decided advantages with respect to the coatings theretofore obtained by the prior art methods, the resulting coatings are nevertheless not entirely satisfactory. Moreover, the process is rather cumbersome in that it requires two separate baths and the time required to obtain the desired coating is relatively long by industrial standards.
Accordingly, it is an object of this invention to protect the surface of magnesium and its alloys from corrosion and environmental attacks and consequent degradation.
It is a further object of this invention to protect the surfaces of magnesium with hard, uniform, adherent, smooth, impervious and corrosion-resistant coating.
It is yet another object of this invention to provide such coated articles of magnesium and its alloys which can be used for decorative applications.
It is also an object of this invention to provide an improved method for anodic coating of magnesium and its alloys.
It is still another object of this invention to provide such an improved method whereby the protective coating on the surfaces of magnesium and its alloys is achieved in a single bath.
It is yet another object of this invention to provide a unique electrolytic solution for anodic coating of magnesium and magnesium alloys.
It is still another object of this invention to provide an electrolytic solution which is a stable composition under the electrodeposition conditions, and which facilitate the formation of the desired coating without the necessity for a prior fluoride treatment of the metal.
The forgoing and other unique features of the electrolytic solution and the process of this invention will be further described, and more fully appreciated, from the ensuing detailed description.